HIGH PRESSURE FUEL INJECTION TUBING MATERIAL AND METHOD OF MANUFACTURING THE SAME

Background of the Invention:

Field of the Invention:

This invention relates to a high pressure fuel injection tubing material to be used generally for fuel feeding lines of diesel engines and to a method of manufacturing the same.

Description of the Prior Art:

In the high pressure fuel injection tubing material in the prior art shown, for example, in "Fuel Injection Tubing» - SAE on pages 21.123 to 21.12k of the SAE Standards and "Testing Technique for Diesel Fuel Injection Systems" - SAE Recommended Practice on pages 2.91 to 24.96 of SAE Journal 969a, the harden¬ ed grain structure produced over the entire tube wall by cold drawing by repeatedly using dies and plugs is uniformly removed %5 by tempering so that the carbon content and hardness distribution (of Hv = about 110 to 120) may be substantially uniform over the entire tube wall and no residual compression stress may be seen in the finally finished tube diameter to be used.

Therefore, due the actual state of the generation of numerous hair cracks on the inside surface with such repeated cold drawing works, the uniform structure by tempering the entire tubing material under the state of the final uniform carbon content distribution and the section structure in which no residual compression stress is present and, on the other hand, as a multiplied effect of the very high internal flow pressure in the recent tendency and the flow pressure variation and vibra¬ tion variation repeated constantly by the operating state of the engine, the inside surface will be eroded by a cavitation, will be reduced in the smoothness and true circularity and will be locally cracked or broken by such erosion at last. At the same - 2 11&8434 time, in order to prevent them and to secure safety, a large diameter tubing material with the increase of the wall thickness is inevitably forced to be used but the effect can not be well expected. Generally no sufficient mechanical property as of an injection tubing material can be secured and therefore the improvement has been constantly hoped for.

Summary of the Invention In accordance with an aspect of the invention there is provided a high pressure fuel injection tubing material comprising: a steel tube having a fine inner bore, said tube having an outer diameter of about 6 to millimeters and a wall thickness of about 2.2 to 12 millimeters, and wherein only the inside surface of said tube includes a hardness layer contiguous therewith and extending radially outwardly a distance of approximately 13% of the wall thickness, said inside surface having a hardness distribution of approximately 150 to 230 HV, whereby the unwanted cavitation and cracking of the inner surface due to high pressure fuel injection is reduced, while substantially maintaining the mechanical properties of the steel tube.

The high pressure fuel injection tubing material according to the present invention is characterized in that the carbon content is gradually increased with the rise of the hardness distribution toward the inside surface from the outside surface of the wall thickness of a fine diameter thick metal tube made of a single steel material and a residual compression stress producing layer substantially in the peripheral dir¬ ection is formed in the inside surface.

Further objects, advantages and features of the present invention will become more fully apparent from a detailed consideration of the arrangement and construction of the constituent parts as set forth in the following specification taken together with the accompanying drawing.

Brief Description of the Drawings:

In the drawing; Figure 1 is a partly sectioned side view of a high pressure fuel injection tubing material according to an embodiment of the present invention.

Figure 2 is a sectioned view on line II-II in Figure 1.

Detailed Description of Preferred Embodiments:

In the drawings, only the inside surface 4 side of the wall thickness 2 of an injection tubing material 1 con¬ sisting of a fine diameter thick wall metal tube of a tube diameter of about 6.0 to 30 mm and wall thickness of about 2.2 to 12.0 mm made of a single steel material is cemented and then the entire tubing material 1 is heated and is then immediately quenched from the outside surface 3 side or, after such cementation, only the inside surface 4 side is quickly heated by high frequency induction _ 4 _ heating or the like and is then left to naturally cool. A compact crystal structure high in the carbon content -with the rise of the hardness distribution making the inside surface 4 have Hv = about 150 to 230 toward the inside surface k from the outside surface 3 side of the -wall thickness 2 of the tubing material and having a residual compression stress producing layer particularly in the peripheral direction including the length¬ wise axial direction can be formed by this treatment.

As regards the production of these residual compression stresses, the tubing material is of two phases consisting of ferrite and pearlite but only the inside surface h: side high in the carbon content with the rise of the hardness distribution is formed to be of two phases consisting of martensife and troostite by the above mentioned treatment and therefore the producing part 5 is produced only on the inside surface k side by the multiplied effect of the expansion difference of these different metal struc¬ tures and the temperature difference upon quenching. Thus, the carbon content with the rise of the hardness distribution is gradually higher from the outside surface 3 side to the inside surface %. Further, as the tubing material is formed of the compact crystal structure having the residual compression stress producing part § in the peripheral direction, the cavitation by the flow pressure on the inside surface k can be effectively prevented, the cracking and breaking with the reduction of the smoothness (roughness) and true circularity of the inside surface by erosion can be prevented, the pressure resistance can be well increased without increasing the tube diameter by increasing the wall thickness and the mechanical properties against cracking and breaking by vibrations or the like can be improved and well secured by the section structure of different metal structures distributed and formed in the inside and outside surfaces of the wall thickness 2, Thus the present invention is very industrially- useful .

While the present invention has been described with reference to particular embodiments thereof, it will be under¬ stood that numerous modifications may be made by those skilled in the art without actually departing from the spirit and scope of the invention as defined in the appended claims- - 6 118434 The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A high pressure fuel injection tubing material comprising:

a steel tube having a fine inner bore, said tube having an outer diameter of about 6 to 30 millimeters and a wall thickness of about 2.2 to 12 millimeters, and wherein only the inside surface of said tube includes a hardness layer contiguous therewith and extending radially outwardly a distance of approximately 13% of the wall thickness, said inside surface having a hardness distribution of approximately 150 to 230 HV, whereby the unwanted cavitation and cracking of the inner surface due to high pressure fuel injection is reduced, while substantially maintaining the mechanical properties of the steel tube.

2, The tubing material according to claim 1 wherein said inside surface is made of two phases consisting of martensite and troostite.